1. Field of the Invention
The present invention relates to a stencil printing apparatus, and more particularly to a stencil printing apparatus that is capable of switching between simplex printing using a simplex master and single-step duplex printing using a duplex master.
2. Description of the Background Art
Digital thermal stencil printing is known as a simple conventional printing method. In a stencil printing apparatus used for this stencil printing, a thermal head on which fine heat-generating elements are arranged in series is brought into contact with a master, and the master is conveyed while electrifying the heat-generating elements in a pulsating fashion such that the master is thermally melt-perforated in accordance with image information. The master is then wrapped around the outer peripheral surface of a perforated cylindrical plate cylinder, whereupon the outer peripheral surface of the plate cylinder is pressed via a sheet of paper using pressing means such as a press roller. As a result, ink is transmitted through the perforated portions of the master and transferred onto the sheet, whereby a printed image is obtained.
In stencil printing, duplex printing, in which printing is performed on both sides of a sheet, is often performed recently with the aims of reducing paper consumption, reducing the amount of space required to store documents, and so on. When duplex printing is performed using a conventional method, a sheet printed on both sides is obtained by conveying a sheet stacked on a sheet feeding unit to a printing unit, where printing is performed on one side of the sheet, turning the sheet over, and then returning the sheet to the printing unit, where printing is performed on the other side. However, this method is problematic in that it is troublesome to reset the sheet in the sheet feeding unit after it has been discharged and align the sheet after printing has been performed on one side. Furthermore, since the sheet passes through the printing unit twice, another problem arises in that duplex printing requires twice the time of simplex printing even in terms of the net printing time, which is excessive.
To solve these problems, a duplex printing apparatus that is capable of obtaining a sheet printed on both sides in a single step has been proposed in Japanese Unexamined Patent Application Publication 2005-246730, for example. In this apparatus, a duplex master on which a first engraved image and a second engraved image are arranged in the rotation direction of a plate cylinder is used. A first sheet is then fed from a sheet feeding unit and one of the engraved images is printed onto the front surface thereof. The sheet is then guided to an auxiliary tray, whereupon a second sheet is fed from the sheet feeding unit and one of the engraved images is printed onto the front surface thereof. The second sheet is then guided to the auxiliary tray, and at the same time, the first sheet is re-fed from the auxiliary tray and the other engraved image is printed onto the rear surface thereof. This sheet is then discharged onto a discharge tray. By performing this operation continuously, a sheet printed on both sides is obtained in a single step.
However, if a plurality of plate cylinders are used alternately when employing the technique disclosed in this publication, and the master wrapped around the plate cylinder has been engraved with duplex printing images, only engraving and simplex printing can be performed, and to print the same image on both sides, engraving must be performed again, leading to an increase in master consumption.